The theory that all failures (all evils) are due to insufficient knowledge. This is the key to the rational philosophy of the unknowable.
Optimism would be futile if there were fundamental limitations to the creation of knowledge, but there are not. It would be futile if there were fields – especially philosophical fields such as morality – in which there were no such thing as objective progress. But truth does exist in all those fields, and progress towards it is made by seeking good explanations.
Problems are inevitable, because our knowledge will always be infinitely far from complete. Some problems are hard, but it is a mistake to confuse hard problems with problems unlikely to be solved. Problems are soluble, and each particular evil is a problem that can be solved.
An optimistic civilization is open and not afraid to innovate, and is based on traditions of Criticism. Its institutions keep improving, and the most important knowledge that they embody is knowledge of how to detect and eliminate errors.
Optimism also infers that we should not fear extra terrestrial civilisations. For them to travel across the vast expanses of space to us necessitates a certain degree of progress. Most likely they would have had to figure out atomic transmution, for example, and would have no need for our resources.